1. Field of the Invention
This invention relates to an active diffraction grating that can be suitably used in an optical switch or the like for high-speed optical communication.
2. Description of the Related Art
Diffraction gratings on conventional semiconductor substrates are prepared by forming grooves at a constant spacing using an optical molding technique or micro-scale device technique, or by direct writing with electron beams.
FIGS. 1A and 1B are enlarged sectional views showing essential parts of such conventional diffraction gratings.
In FIG. 1A, a substrate 1 is made of a material such as metal or ceramics, and stripe-like recesses and protrusions 2 are formed on the substrate 1 by photolithography and etching.
FIG. 1B shows another conventional example. In this example, triangular recesses and protrusions 3 having a triangular cross section are formed on the top of a semiconductor substrate 1a by similar techniques.
The conventional techniques for preparing stripe-like recesses and protrusions or recesses and protrusions having a triangular cross section on the top of the metal or ceramic substrate 1 or the semiconductor substrate 1a are described in the following references:
Patent Reference 1: JP-A-7-173649
Patent Reference 2: JP-A-8-320506
However, in the diffraction gratings prepared by these methods, the spacing between grooves is constant, and the positions where the diffraction gratings are formed are fixed with respect to the direction of incident light beams. Therefore, there is a problem that these diffraction gratings cannot cope with changes in incident wavelength and changes in incident angle.